s4:getopt.py - set the password callback only when no password has been provided

s4:getopt.py - set the password callback only when no password has been provided

Previously the "no_pass" and "no_pass2" variables weren't handled correctly.
Since at the initialisation of the "CredentialsOptions" we don't have any
password at all. Only afterwards we could get one through "set_password".

If a password is specified, use it. If no password is specified, consider the
use fo an input mask on STDOUT. But if the loadparm context contains one prefer
it over the input.